The invention relates to articles, such as, but not limited to, turbine engine components, that have high yield strength and time-dependent crack propagation resistance. More particularly, the invention relates to articles that have high yield strength and time-dependent crack propagation resistance and are formed from nickel-based superalloys. Even more particularly, the invention relates to nickel-based superalloys that are used to form articles, such as turbine engine components, that exhibit both high yield strength and time-dependent crack propagation resistance.
During operation of jet and land-based turbine engines, high temperatures and stresses are normally encountered. In order to function properly over extended periods of time, the components within these turbine engines must retain high strength and other properties at temperatures in excess of 850° F. Nickel-base superalloys have long been recognized as having properties at elevated temperatures that are superior to those of steel-based components, such as turbine wheels, and which meet the performance requirements of turbines. Precipitates of a γ″ (“gamma double prime”) phase are believed to contribute to superior performance of many of these nickel-base superalloys at high temperatures. Consequently, nickel-base superalloys such as Alloy 706 have been widely used to form components in turbines that are used for land-based power generation.
Newer turbine engine designs have imposed even more demanding requirements upon the properties of materials that are used to form components. In addition to higher operating temperatures and stresses than those encountered in previous designs, the newer turbine engines can present a different operating environment that is potentially more aggressive than that of earlier turbines. One example of a more aggressive operating environment is the use of steam to cool hot gas path materials in the current generation of power turbine engines. Thus, materials having improved properties are needed to deliver a performance level that was not contemplated in the previous generation of turbine engines.
Turbine engine components, as well as other articles, formed from nickel-base superalloys can be subjected to time-dependent propagation of cracks that are either incipient or formed during fabrication or use of the component. Time-dependent crack propagation depends on both the frequency of stress application and the time spent under stress, or “hold-time.” A discussion of the dependence of crack propagation upon frequency and hold time can be found in U.S. Pat. No. 5,129,969 issued Jul. 14, 1992, to M. Henry and assigned to the same assignee as the present application. Because such cracks tend to grow while the component is under the stress of turbine engine operation and can lead to catastrophic failure of the component as well as the entire turbine engine, it is desirable that a component possess a certain level of time-dependent crack propagation resistance (TDCPR) at its service temperature. The TDCPR of an alloy or an article formed from the alloy can be expressed in hours to failure at a given temperature and fracture mechanics driving force.
During operation, gas turbine discs are subjected to large radial temperature gradients. In particular, land-based gas turbine engines operate with long hold times at high temperature. For these applications, strength properties can dominate and drive the bore design, whereas resistance to time-dependent crack growth can dominate the rim design. Turbine wheels or discs must therefore possess adequate time-dependent crack propagation resistance in the rim regions of the wheel at one temperature and adequate tensile strength at a second, lower temperature in the area surrounding the bore of the wheel. It is therefore desirable that the turbine wheels be formed from a material that provides the necessary combination of TDCPR and strength at high temperatures.
The nickel-base superalloys that are either being used in current turbines or are being considered for use in proposed turbine engine designs do not possess the necessary combination of crack propagation resistance and strength. Alloy 718, for example, has been chosen as a turbine wheel material due to its acceptable TDCPR in the steam environment of current turbine designs, but its TDCPR could be inadequate in more advanced designs. Alloy 625 has excellent crack propagation resistance, but has insufficient strength for turbine wheel applications. Commercially available alloys such as ASTROLOY™ have good combinations of TDCPR and strength when the material is processed to form articles that are sized small enough to be cooled quickly—i.e., at rates between about 150° F. and about 600° F. per minute—from the solutioning temperature. When processed on the scale of modern land-based gas turbine wheels, however, such alloys have inadequate strength. This is due in part to the fact that the alloy that is obtained is a γ′ (“gamma prime”) strengthened alloy rather than a γ″ (“gamma double prime”) strengthened alloy. The γ′ strengthened alloy exhibits accelerated precipitation kinetics.
As their operational parameters are extended, both land-based and jet turbine engines will need to incorporate components that are formed from materials that possess the time dependent crack propagation resistance and strength required for these applications. Therefore, what is needed is an article, such as a turbine engine component, that possesses adequate time dependent crack propagation resistance at high temperatures. What is also needed is an article that possesses a combination of time dependent crack propagation resistance and strength at high temperatures. What is further needed is a nickel-base superalloy that can be formed into an article, such as a turbine engine component, having the necessary combination of TDCPR and strength at high temperatures.